In the case where one of three optical cables connected to a submarine branching unit forming an optical submarine cable system suffers a breakdown and can no longer be used, it is desirable to keep the submarine cable system in service through maximum utilization of communication by the other remaining two optical cables. To this end, the optical path connected to the failed optical cable needs to be switched to an optical path connecting other optical cables.
FIG. 1 is a block diagram depicting the construction of an optical path switching circuit that is a prior art example 1 disclosed in Japanese Patent Application Laid-Open Gazette No. 164022/98. In FIG. 1, reference numeral 101 denotes generally an optical switching circuit; 102 denotes a first input port; 103 denotes a first output port; 104 denotes a second input port; 105 denotes a second output port; 106 denotes a third input port; 107 denotes a third output port; 108 to 110 denote first to third preceding optical couplers; 111 to 113 denote first to third succeeding optical couplers; 114 to 116 denote first to third optical filters that permit the passage therethrough of only control signals of predetermined wavelengths; and 117 to 119 denotes first to third optical switches.
Next, the operation of the prior art example will be described below.
An optical signal input via the first input port 102 is branched by the first preceding optical coupler 108 to the first succeeding optical coupler 111 and the first optical filter 114.
When the input optical signal from the first input port 102 contains no control signal, the first optical filter 114 yields no output, and the first optical switch 117 sends an optical signal input via the third input port 106 to the first output port 103.
On the other hand, when the signal from the first input port 102 contains a control signal, the control signal having passed through the first optical filter 114 is input to the first optical switch 117, which sends, in turn, an optical signal input via the second input port 104 to the first output 103.
Similarly, when the optical signal from the second input port 104 contains no control signal, the second optical switch 118 sends the optical signal input from the input port 102 to the second output port 105. When the optical signal input from the second input port 104 contains a control signal, the second optical switch 118 sends the optical signal input from the third input port 106 to the second output port 105.
Likewise, when the optical signal from the third input port 106 contains no control signal, the third optical switch 119 sends the optical signal input from the second input port 104 to the third output port 107. When the optical signal from the third input port 106 contains a control signal, the third optical switch 119 sends the optical signal input from the first input port 102 to the third output port 107.
FIG. 2 is a block diagram depicting the construction of an optical path switching circuit that is a prior art example 2 set forth in Japanese Patent Publication Gazette No. 83141/94. In FIG. 2, reference numeral 121 denotes generally an optical path switching circuit; 122 to 124 denote first to third input/output ports; 125 and 126 denote first and second input ports of the first input/output port 122; 127 and 128 denote first and second output ports of the first input/output port 122; 129 and 130 denote third and fourth input ports of the second input/output ports 123; 131 and 132 denote third and fourth output ports of the second input/output ports 123; 133 and 134 denote fifth and sixth input ports of the third input/output port 124; 135 and 136 denote fifth and sixth output ports of the third input/output port 124; 137 to 142 denote first and sixth main optical paths; 143 to 148 denote first to sixth optical switches placed in the first to sixth main optical paths; 149 denotes a first switching optical path that interconnects the first and fourth optical switches 143 and 146; 150 denotes a second switching optical path that interconnects the fourth and sixth optical switches 146 and 148; 151 denotes a third switching optical path that interconnects the second and third optical switches 144 and 145; and 152 denotes a fourth switching optical path that interconnects the third and fifth optical switches 145 and 147.
Next, the operation of the above prior art example will be described below.
An optical signal input via the first input port is sent over the second main optical path 138 to the third output port 131. An optical signal input via the second input port 126 is sent over the sixth main optical path 142 to the sixth output port 136. An optical signal input via the third input port 129 is sent over the first main optical path 137 to the first output port 127. An optical signal input via the fourth input port 130 is sent over the third main optical path 139 to the fifth output port 135. An optical signal input via the fifth input port 133 is sent over the fourth main optical path 140 to the fourth output port 132. An optical signal input via the sixth input port 134 is sent over the fifth main optical path 141 to the second output port 128.
In the event of failure on the part of the second input/output port 123, the optical signal input via the fifth input port 133 can be sent over the first switching optical path 149 to the first output port 127 by switching the first and fourth optical switches 143 and 146. Further, the optical signal input via the first input port 125 can be sent over the second switching optical path 151 to the fifth output port 135 by switching the second and third optical switches 144 and 145.
In the event of failure on the part of the third input/output port 124, the optical signal input via the second input port 126 can be sent over the second switching optical path 150 to the fourth output port 132 by switching the fourth and sixth optical switches 146 and 148. Further, the optical signal input via the fourth input port 130 can be sent over the fourth switching optical path 152 to the second output port 128 by switching the third and fifth optical switches 145 and 147.
In the event of failure on the part of the first input/output port 122, the optical signal input via the third input port 129 can be sent over the first and second switching optical paths 149 and 150 to the sixth output port 136 by switching the first and sixth optical switches 143 and 148. Further, the optical signal input via the sixth input port 134 can be sent over the fourth and third switching optical paths 152 and 151 to the third output port 131 by switching the second and fifth optical switches 144 and 147.
Since the optical path branching circuit of the prior art example 1 has such a construction as described above, it is necessary that optical signals be equally distributed to the succeeding optical couplers—this gives rise to the problem of a loss of about 3 dB (50%) inflicted on the optical signal that is output from each output port.
Further, since the optical path branching circuit of the prior art example 2 has such a construction as described above, four optical switches need to be switched in the case where when a failure occurs on the part of any one of three input/output ports, the failed input/output port is disconnected so as to obtain maximum utilization from communications between the remaining input/output ports. Further, each optical switch needs to operate in the event of failure on the part of two input/output ports. This leads to the problem of complexity in the configuration of an optical switch control circuit.
The present invention is intended to solve such problems of the prior art as referred to above, and has for its object to provide an optical path switching circuit that permits reduction of the loss of the output optical signal from each output port and simplification of the optical switch control circuit.